This invention relates to semiconductor laser devices capable of effectively transmitting heat away and in particular to such semiconductor laser devices adapted for high-frequency operations.
Semiconductor laser devices incorporating a semiconductor laser element as a pick-up light source for a writable disk drive or the like have been known. Such a semiconductor laser element emits laser light as an electric current is caused to flow across its internal p-n junction, but a large amount of heat is also generated at the same time. In order to efficiently remove this heat, such a laser element is commonly mounted with its p-surface downward (according to the so-called "junction-down" mounting format) to a silicon submount with a high thermal conductivity such that the generated heat is quickly conducted away therethrough. This mounting format is considered advantageous because the distance between the p-n junction which is the source of the generated heat and the submount can be reduced with the p-surface of the laser element facing downward.
FIGS. 9 and 10 show a prior art semiconductor laser device structured in this manner. Three externally extending lead pins 15, 16 and 17 (referred to for convenience as "the first", "the second" and "the third", respectively) are attached to a disk-shaped member 10 (referred to as "the stem"), the first pin 15 being directly attached to the stem 10 so as to be electrically connected therewith, while the second and third pins 16 and 17 are each affixed to the stem 10 by way of an insulator 18. A heat sink 20, which may comprise a material such as Cu, is soldered to a main surface of the stem 10 and an electrically conductive silicon submount 25 is attached to the heat sink 20. On the surface of the submount 25, there is not only an Al wiring pattern 40 formed through an oxide layer 41 comprising SiO.sub.2 but also an Al pad formed directly.
A semiconductor laser element 30 is deposited on this Al wiring pattern 40 in the aforementioned junction-down format such that the heat generated thereby can be efficiently conducted away therefrom. The n-surface of the laser element 30 and the Al pad 35 are electrically connected by a metallic wire 36.
The second lead pin 16, which serves to supply power therethrough to the laser element 30, is extended internally to a position near the submount 25 and is electrically connected to the Al pattern 40 by another metallic wire 42.
A light-receiving element 50, which serves to receive the laser light emitted backwards from the laser element 30 and to thereby monitor its optical output, is directly mounted to the main surface of the stem 10, and its upper surface is electrically connected to the third lead pin 17 by still another metallic wire 51. All these components described above, inclusive of the laser element 30, are sealed inside a cap (not shown in FIGS. 9 and 10) to form a packaged product.
When a semiconductor laser device thus formed is used for an optical disk, for example, the heat sink 20 and the second pin 16 respectively serving as the negative electrode and the positive electrode, a current flows from the second pin 16 sequentially through the metallic wire 42, the Al pattern 40, the p-surface of the laser element 30, its n-surface, the wire 36, the Al pad 35 and the submount 25 to the heat sink 20, as shown by arrows, such that laser light is emitted from the laser element 30.
It is to be noted, regarding the prior art semiconductor laser device described above, that the Al pattern 40, on which the laser element 30 is deposited, and the electrically conductive submount 25 must be separated from each other by the electrically insulating oxide layer 41 both because the laser element 30 must be mounted to the submount 25 in the junction-down format and because the heat sink 20 must be used as the negative electrode. Since the thermal conductivity of the oxide layer comprising SiO.sub.2 is 1.4-7.2 W/m.degree. K and is much smaller than that of the silicon submount 25 (about 150 W/m.degree. K), this means that the heat generated by the laser element 30 cannot be efficiently conducted away to the submount 25.
In view of the above, it has also been known to use a submount made of a material other than silicon such as AlN that is electrically insulating but has a larger thermal conductivity (160-200 W/m.degree. K) than silicon. FIG. 11 shows another prior art semiconductor laser device characterized (and distinguishable from the example shown in FIGS. 9 and 10) as having a submount 25' made of electrically insulating AlN attached on top of a heat sink 20 comprising Cu or the like. Two Al wiring patterns 40' and 35' are formed on the surface of the AlN submount 25' such that they are electrically separated from each other, and a laser element 30 is deposited on the Al pattern 40' in the junction-down format. The n-surface of the laser element 30 and the Al pattern 35' are connected to each other electrically by a metallic wire 36a, and the Al pattern 35' and the heat sink 20 are connected electrically to each other by another metallic wire 36b. The Al pattern 40' is also electrically connected through still another metallic wire 42 to a lead pin 16 for supplying a current from an external source (not shown). With a semiconductor laser device thus structured, a current flows from the pin 16 sequentially through the wire 42, the Al pattern 40', the p-surface of the laser element 30, its n-surface, the wire 36a, the Al pattern 35' and the wire 36b to the heat sink 20, as shown by arrows, such that laser light is emitted from the laser element 30.
The semiconductor laser device described above with reference to FIG. 11 can therefore be cooled more efficiently because the heat generated by its laser element 30 can be conducted off to the thermally conductive AlN submount 25' only through the Al pattern 40'. The use of a submount made of AlN, instead of silicon, however, has the following practical problem.
When semiconductor laser devices are produced, screening tests therefor for quality control are not easy to carry out if they are to be carried out only on the laser elements because semiconductor laser elements are extremely small. Thus, screening tests are usually carried out after the laser elements are each deposited on a submount. In other words, a test on electrical and optical characteristics of each laser element is carried out not on the laser element alone but on the combination consisting both of the laser element and also of the submount on which it is deposited. If the test on a combination shows that an adjustment is required, such an adjustment is made, say, by a so-called burn-in process, and the adjusted combination is then attached to a heat sink. If the test shows that it is not adjustable, however, the combination is discarded as a whole. In summary, if a laser element is unadjustably defective, the submount to which it is mounted is also discarded. Since silicon submounts are relatively inexpensive, the procedure described above is not impractical, not incurring a serious economical loss. Since AlN submounts are significantly more expensive (say, by a factor of several tens) than silicon submounts, the loss due to discarded AlN submounts can significantly affect the production cost of the laser devices.
It is therefore an object of this invention to provide a semiconductor laser device from which generated heat can be effectively removed although a silicon submount is used for mounting a semiconductor laser element thereto.
Another problem with prior art semiconductor laser devices as described above with reference to FIGS. 9, 10 and 11 is that their submount serves as a capacitor (with capacitance C1) and its package contributes an inductance L1 when they are operated such that their equivalent circuit diagram may look as shown in FIG. 12. It now goes without saying that such capacitance C1 and inductance L1 effectively prevent a high-frequency operation of the device.
It is therefore a further object of this invention to provide a semiconductor laser as described above which can also be adapted for high-frequency operations.